The present invention relates to a radial ply tire for heavy duty vehicles having an improved bead durability.
Radial ply tires have been widely used for heavy duty vehicles such as trucks, buses and the like for increase in running speed due to the recent development and extension of highway networks and, in particular, for increase in tire load due to the recent large-sized vehicles such as construction vehicles.
In comparison with a bias ply tire, a radial ply tire has less rigidity at the sidewall portions due to its carcass ply construction, and accordingly such portions have more deformation. As a result, the bead portions are also repeatedly subjected to large deformation when running.
In the tires used with dump trucks under heavy loads and in the tires used with refrigerated trucks having a high gravitational center, strain in the bead portions becomes especially large, and a structural failure in the bead portion has sometimes been caused.
In the case where the carcass is composed of steel cords arranged radially of the tire and turned up at the edges thereof around bead cores from the inside to the outside thereof, structural failure is apt to be caused at the upper edge of the turned up portions of the carcass by the stress on the above-mentioned upper edges, which stress separates the cords from the surrounding rubber.
Such rubber separation at the upper edge of the carcass turned up portion has been known as a main cause for bead failure.
In order to prevent such separation failure, for example as shown in FIG. 3, a reinforcing layer C having an inner portion C1 and an outer portion C2 extending radially outwardly from the bead base region A along the axially inner main portion B1 and the axially outer tuned up portion B2 of the carcass B, respectively, has been often employed.
On the other hand, for prevention of the rubber separation at the upper edges of the carcass turned up portion, it is more effective to provide reinforcement on the axially inner side of the bead portion to increase the rigidity of the bead portion rather than to dispose a reinforcing layer axially outside the carcass turned up portion to directly cover the upper edge thereof.
Accordingly, the tire shown in FIG. 3 which is provided on the axially inner side of the bead portion with the above-mentioned axially inner portion C1 extending along the main portion B1 of the carcass B meets this requirement.
Such tire, however, sometimes has rubber separation at the upper edge of the axially inner portion C1 earlier than the above-mentioned rubber separation at the upper edge of the carcass turned up portion B2 in particular when the reinforcing layer is composed of only one ply.
Incidentally, if the axially inner portion is not formed on the reinforcing layer, the rubber separation at the upper edge of the carcass turned up portion becomes increased.
In order to prevent the rubber separation at the upper edge of the axially inner portion of the reinforcing layer, various studies were made, and it was found that, in conventional tires as shown in FIG. 4, the thickness of the rubber inner liner D becomes uneven and becomes very thin at the upper edge C1a of the axially inner portion C1, and this is a main cause for the separation thereat.
It is therefore, an object of the present invention to provide a radial ply tire for heavy duty vehicles, in which rubber separation at the upper edge of the axially inner portion of the bead reinforcing layer is prevented, and the durability of the bead portions is improved by maintaining the thickness of the inner liner at a constant thickness.